“Origin of appinitic pockets in the diorites of Jersey, Channel Islands”

Isolated pockets of pegmatitic appinite characterized by hollow-shell, prismatic amphiboIes are
common in the Pre-Cambrian metagabbros and metasomatic diorites of Jersey. Field relationships and petrography indicate a liquid phase in the formation of these appinitic pockets, which are chemically distinct from the
associated gabbros and diorites. Close chemical ties between appinites and host rocks, however, prove a replacive, metasomatic, rather than intrusive origin for the pockets. Significant enrichment in SiOz, KzO, and Na~O suggest that sunounding granite provided the metasomatic agents. The localized changes in composition of the basic rocks resulted in the formation of partially molten pockets from which the appinites crystallized. This mechanism probably necessitates a temperature in the region of 9oo ~ at 2-5 Kb PH20 : Fractured, hollowshell, prismatic amphiboles of the pockets are consistent with quench crystallization, possibly due to the sudden loss of volatiles. An increase in the oxygen fugacity may have played a major role in inducing the rapid crystallization of kaersutitic amphibole. The envisaged conditions tinder which these changes took place are those of a high-level, sub-volcanic environment.